The remedy of a malignant tumor by the radiation of focused ultrasound from the outside of a living body is less invasive, compared with surgery and is excellent in principle in relation to the deterioration of the physical strength during an operation of a patient, the recovery after the operation and the enhancement of the quality of life. Therefore, in future, the social value is considered to be enhanced.
One of remedies using focused ultrasound uses thermal effect that ultrasonic energy by a radiated ultrasonic wave is absorbed by a living tissue and hereby, the temperature of a disease region rises. The other one uses chemical interaction between a chemical substance existing in a disease region and a radiated ultrasonic wave. Of the latter, a remedy using a substance that generates active oxygen when an ultrasonic wave is radiated is proposed by Umemura and others and is named a sonodynamic therapy. Acoustic cavitation is considered to play an important role in a mechanism in which therapeutical effect is obtained by the sonodynamic therapy.
For a method of efficiently causing and collapsing acoustic cavitation for this purpose, heretofore, only physical means have been proposed. In a U.S. Pat. No. 2,741,907, technique for switching a sound field at an interval of 0.01 to 10 milliseconds (ms) and radiating an ultrasonic wave is reported. According to this technique, an ultrasonic wave is radiated, switching sound fields different in a wave front at an interval of the above time in consideration of a fact that ultrasonic irradiation time required for causing acoustic cavitation is 0.01 to 10 ms and a cycle that acoustic cavitation caused by one sound field is collapsed by the other sound field is repeated. Hereby, the efficiency of sonochemical reaction can be improved by an order of magnitude with the same ultrasonic power, compared with a case that sound fields are not switched.
Also, in the U.S. Pat. No. 5,523,058, technique in which an ultrasonic wave having a waveform advantageous for causing acoustic cavitation, which normally obtained only in the presence a reflector, without an reflector is disclosed. In the technique, a waveform advantageous for causing acoustic cavitation is acquired by superimposing its double frequency on one frequency component. As the effect of a reflector cannot be necessarily expected when an ultrasonic wave is radiated on a living body, the technique is expected to contribute to the increase of the effect and the enhancement of safety in remedy by ultrasonic irradiation.
Also, in Japanese published examined patent application No. Hei6-29196, for a method of chemically enhancing the anti-tumor effect of an ultrasonic wave, a method of using a substance that generates active oxygens by the chemical effect of ultrasound exposure is reported. A substance such as porphyrin used in the technique has a function of secondarily generating active oxygen by acoustic cavitation caused by an ultrasonic wave, however, the substance could not reduce the threshold of cavitation. In the meantime, in WO98/01131, a method of reducing the threshold of cavitation by an amphiphilic xanthene dye sensitizer and secondarily generating active oxygen by acoustic cavitation caused by an ultrasonic wave is proposed.
The social evaluation of minimally invasive remedy in which therapeutical effect is acquired by causing acoustic cavitation using focused ultrasound is considered to increase in future. In case an ultrasonic wave is used, there is no problem that a patient is exposed, compared with other less invasive remedy such as a heavy ion particle radiotherapy and radiation therapy and the miniaturization of an apparatus is easy. Therefore, the use by a patient himself/herself except a hospital under the suitable direction of a doctor such as home remedy may be capable.
However, home remedy using a conventional type apparatus using a convergent ultrasonic wave has a problem related to focusing. That is, as known from the above well-known examples, a conventional type apparatus using focused ultrasound premises that after a diseased part and its state are specified by diagnosis based upon images and biopsy, a convergent ultrasonic wave is focused on the disease region and is precisely radiated. This reason is that when the convergent ultrasonic wave is focused off the disease region, a non-disease region of a human body is damaged. To radiate a precisely focused ultrasonic wave, a medical apparatus for imaging disease regions is required. However, currently, no medical apparatus is available which is small enough for home use and at the same time capable of targeting disease regions. Further, to decide the location and the state of a disease region based upon images by the current medical image diagnostic apparatus, expert knowledge is required and non-expert patients cannot target a disease region.